Zen has been popular in America since the late 1940s when D.T. Suzuki came to America and began to write about Zen for Westerners.
Other Zen masters and aficionados have made Zen concepts popular and accessible to Westerners through the last five decades, including Shunryu Suzuki, R.H. Blyth, Jack Kerouac, and Robert Pirsig, the author of "Zen and the Art of Motorcycle Maintenance".
Feminism, psychology, and Zennism (the practice of Zen minus the practice of Buddhism) have all influenced American conceptions of Zen.
You don't need to be Buddhist or any particular religion to practice Zen, but Zen can inform and enlighten any spiritual path.